ideally you want around 2 days for starter to be fully ready sometimes as long as 3 days . You will have a foamy head but not much , you also should cover with foil and not a bung/airlock as you want oxygen to get in for the yeast and it is best to give your starter a swirl every few hours or so to help get more oxygen for the yeast , the yeast needs time to multiple and to keep it in its growth stage an og of around 1030/1040 is wanted so the yeast stays in the growth stage and not converting to alcohol stage . If like me you want to strain the yeast but need a starter then pitch early like after 1 day if making brews like hefes .